Transmission will not shift
There are many components in a modern transmission and all need to work in harmony, below is a web site that can you you more insight on how a transmission works. Your car needs to go to a transmission shop to be tested, I'm sure its something simple. http://www.mistertransmission.com/transmission -components
Make sure your fluid lever is ok...
My 96 s10 Blazer did the exact same thing. I got it checked out and it had 2 transmission codes pop up. One was P1860 (TCC PWM Solenoid Circuit Electrical) and the other was p1870 (Transmission Component Slipping). I replaced the torque converter, inner and outer wire harnesses, and the valve body. It shifts perfect now.
Looking for a Used Sierra 1500 in your area?
CarGurus has 64,898 nationwide Sierra 1500 listings starting at $999.
Search GMC Sierra 1500 Questions
GMC Sierra 1500 Experts